Posting a message to the forum will remove the above advertisement
  1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.
  2. Checkout our Plumbers Insurance area - heavily discounted Public Liability Insurance and Van Insurance specifically designed for plumbers.

Discuss Way to work out flow rates in the Plumbing Forum area at

Thread Status:
Not open for further replies.
  1. southcoastboile

    southcoastboile Plumber GSR

    does anyone have a chart or know an equation to work out how pressure will improve over the length of the pipe.

    We have installed an unvented cylinder in a loft and have 14lpm with water softner connected. 18lpm without. 2.2bar pressure at shower upstairs.

    Customer wants more pressure as they have 25lpm at tap downstairs. 4.5bar pressure. Confirmed by water board apparently.

    The main runs up to the cupboard in 15mm so we are looking to find out what the lpm gain would be if we were to upgrade the 15mm pipe to 22mm.

    I wondered if anyone had a sum to work it out. I.e every mtre in height of 22mm pipe will reduce the flow rate from at the tap by 0.5lpm or something like that?

    Many thanks in advance!
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Can the water softener do 25lpm ??
  3. southcoastboile

    southcoastboile Plumber GSR

    I'm not to sure to be honest.

    I will see if I can find out the make and model of the water softner and see what max flow rate it can achieve but it would be useful to be able to go back to the customer and say, in theory, if water softner can cope or it was bypassed, We can upgrade the 15mm pipe and the lpm will improve by such and such.

    I've looked online for a way to work it out but I can't find anything simple?
  4. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    If the incoming is 25lpm

    How many elbows and pulled bends

    Is the feed direct eg just for the cylinder


    Etc ??
  5. southcoastboile

    southcoastboile Plumber GSR

    Incoming is 25lpm.

    I think about 6 elbows in total. Mains feeding unvented cylinder only. Currently in 15mm.

    Unvented cylinder is in the loft at the moment of a normal sized house.
  6. rpm

    rpm Trusted Plumber

    When you say pressure is that static or dynamic? Was at a house recently where static was 6bar & dynamic was 0.6bar.
    • Like Like x 1
  7. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    22mm will be more than good enough

    Try and do as many pulled bends as pos tho
  8. southcoastboile

    southcoastboile Plumber GSR

    25lpm and 4.4bar I believe this is static and dynamic pressure?

    Customer wants to know what gain they will get by upgrading the pipe that's the problem. I need a formula if there I one to give a rough idea as to what the gain may be.
  9. steadyon

    steadyon Active Member

    1. Have a look at
    2. Measure the customers incoming flow rate and pressure for yourself - if the water board have measured it, it may not be at the property but at the meter.
    3. You could always disconnect the output side of the softener and measure the maximum (open pipe) water flow, but you'd need to be sure there were no restrictions on the input side.
    4. With respect to your post #3, You might want to suggest to the client that "altering xxxx to yyyy might in theory result in zzzz". I'd be very careful about making any direct promises
    • Agree Agree x 1
  10. rpm

    rpm Trusted Plumber

    Dynamic is always lower than static pressure.
  11. scott_d

    scott_d Plumber GSR

    Have you done your own tests or just going on the information from customer/waterboard?
  12. Last Plumber

    Last Plumber Plumber GSR Top Contributor!!

    You need to look further into that. There's too big a difference.
    If you are measuring 4.5 downstairs in a 'normal house' I would be expecting 4.0 in the loft, or there and there about's. How can it have lost 2.3 bar ?

    How did you measure the upstairs pressure?
  13. Last Plumber

    Last Plumber Plumber GSR Top Contributor!!

    Just read the original post again.

    If you are measuring the Shower water pressure, (presume this is off the new cylinder)? You are measuring it after the Governor/Regulator.

    If you increase the pipe size from 15 - 22 you will reduce the frictional losses. To calculate the amount you need to know route, distance, how many bends/elbows etc.

    To measure the pressure loss through the supply to the cylinder you would need to know the Dynamic Pressure before the water governor on the cylinder and the same at the stop tap at the same flow rate.

    I don't know if this helps?
  14. Ric2013

    Ric2013 Plumber Top Contributor!!

    No. That's flow and pressure. At a guess, I'd say the 4.4Bar is the static pressure.
    • Agree Agree x 1
  15. southcoastboile

    southcoastboile Plumber GSR

    Thanks rik. That's what I meant sorry. Just like standing and working pressure at a meter.

    Thanks for reply last plumber. So let's say we know the dynamic pressure at the stopcock and before the pressure reducing valve. What calculation would you then use to work out what you would potentially gain by upgrading the pipe?
    Its okay knowing pressures prior to the cylinder pressure red valve and at the stopcock but they won't be the same due to the height the water is traveling and the restrictions in the pipe work. I.e bends and sockets.
    I would assume that pressure at stopcock and flow rate would be all that was needed to work out what pressure you would achieve the other end of the pipe.

    For example 25lpm and 4 bar at stopcock would reduce pressure by 0.2lpm for every mtre of horizontal pipe. 0.4 every meter of vertical. 0.2 for every bend, 0.1 for every socket.

    Surely someone must have come up with a simple equation like that?
Similar Threads - flow rates Forum Date
best combi boiler to replace aging worcester 35cdi without losing flow rates Plumbing Forum Nov 14, 2018
Underfloor heating - supply & return flow temperature question Central Heating Forum Monday at 5:21 PM
Thread Status:
Not open for further replies.